The demand for broadband multimedia communication services such as Internet and video delivery is explosively increasing. Accordingly, in trunk line system and metro system, introduction of a higher capacity and higher reliability dense wavelength-division multiplexing optical fiber communication system capable of longer distance transmission is in progress. Further, in subscriber system also, proliferation of optical fiber access services is rapidly growing. In such communication systems using an optical fiber, it is required to enhance the transmission bandwidth efficiency for each optical fiber. Further, in order to reduce the environmental burden, it is required that communication systems using an optical fiber have a structure to achieve a small size, lower power consumption and low cost.
In order to enhance the transmission bandwidth efficiency for each optical fiber, it is necessary to increase the data symbol frequency or increase the multilevel number.
In order to achieve operation with increased data symbol frequency, the bandwidth of elements is a critical factor. Taking optical elements (such as an optical modulator and a photoreceiver, for example) used generally in optical fiber communication systems as an example, the bandwidth is limited for CR time constant mainly controlled by the effect of the capacitance of elements. Because those optical elements use the interaction between light and electricity, the required voltage and element capacitance are determined by the electric field strength and the interaction length. In general, when the interaction length is long, the electric field strength per unit length can be weak, but the element capacitance increases. Therefore, in an optical modulator or an optical switch, for example, power consumption and extinction characteristics, and bandwidth are in the relationship of trade-off. Further, in a receiver, receiving sensitivity and bandwidth are in the relationship of trade-off. Thus, in those optical elements, it has been necessary to make eclectic design in consideration of such trade-off relationship.
To address the above issue, a technique to solve the trade-off between the interaction length and the bandwidth has been proposed. As an example, the traveling-wave electrode structure that avoids bandwidth degradation by making the propagation speed of light and electricity closer is proposed (Patent Literature 1). Further, the segmented-electrode structure that reduces the capacitance by electrically dividing a long electrode and driving them independently or the like is proposed (Patent Literatures 2 to 4).
Further, a method for increasing the multilevel number and reducing the environmental burden is proposed. As such a method, a structure that performs operation on an optical signal as it is to reduce the burden of electrical signal processing is proposed instead of a general method that generates a complicated electrical signal format and then converts it into an optical signal. As such a method, the optical modulator that can generate a quadrature amplitude modulation (QAM) signal by placing optical waveguides that control the phase or amplitude of light in parallel and multiplexing optical signals is proposed (Patent Literature 5). Further, the optical modulator that can generate a phase shift keying signal by dividing and placing regions to control the phase or amplitude of light along the propagation direction on one optical waveguide or the like is proposed (Patent Literature 6).
Further, the structure which connects a plurality of independent optical modulators in series or in parallel and in which phase modulation or intensity modulation is performed in each of the optical modulator is proposed (Patent Literature 7). According to this structure, it is possible to correct delay variations between different bits by superimposing modulated light along the phase axis or time axis.